The present invention relates to a circuit for monitoring an inductive circuit which is formed as part of a signal-processing circuit or as an additional element and which is connected, through a high-ohmic input circuit or filter circuit, to the signal-processing circuit. The monitoring initiates a test cycle to determine the inductance of the circuit to be monitored, including the filter circuit, if predetermined conditions apply and/or in predetermined intervals.
A monitoring circuit of this type is disclosed in EP 0 358 887 A1. In this monitoring circuit, the duration of a test signal passing through the inductive circuit to be monitored is monitored and evaluated with respect to proper duration by means of a time-difference measuring apparatus. To this end, the time difference of the duration of a signal, which is conducted to the time-difference measuring apparatus through the inductance and a time element, is compared to the signal which is conducted directly through an equal time element to the measuring apparatus.
Circuits of this type are particularly suitable to detect short circuits in the inductive transducer of a wheel sensor. If the short circuit is in the line leading to the transducer, in its input circuit or filter circuit, the short circuit is likewise detected in the monitoring operation. A line interruption is also detected. Sensors of this type, which are required in anti-lock systems or traction slip control systems of automotive vehicles, for example, are safety-critical component parts which should be checked permanently for operability, short circuits or line interruptions. In a low-ohmic input circuit or filter circuit, a short circuit may be detected relatively easily by determining the ohmic resistance between an output of the filter circuit and ground. For a high-ohmic filter circuit, such an arrangement is not suitable in practical operations because the ohmic internal resistance of the inductive circuit is low compared to the resistors in the circuit or input circuit. Therefore, the voltage drop, which may be measured at the output of the filter circuit when a current is applied, will be changed by a short circuit only to such a minor extent that reliable evaluation of the measurement results is not possible.